High efficiency quantum dot sensitized thin film solar cell with absorber layer

ABSTRACT

A photovoltaic (PV) device having a quantum dot sensitized interface includes a first conductor layer and a second conductor layer. At least one of the conductor layers is transparent to solar radiation. A quantum dot (nanoparticle) sensitized photo-harvesting interface comprises a photo-absorber layer, a quantum dot layer and a buffer layer, placed between the two conductors. The absorber layer is a p-type material and the buffer layer is an n-type material. The quantum dot layer has a tunable bandgap to cover infrared (IR), visible light and ultraviolet (UV) bands of solar spectrum.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/344,163, filed Jan. 5, 2012, entitled HIGH EFFICIENCY QUANTUM DOTSENSITIZED THIN FILM SOLAR CELL WITH ABSORBER LAYER, the entiredisclosure of which is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor-based photovoltaic energyconverters, also known as “solar cells,” and to the design andfabrication of the same.

BACKGROUND OF THE INVENTION

With appropriate electrical loading, photovoltaic solid statesemiconductor devices, commonly known as solar cells, convert sunlightinto electrical power by generating both a current and a voltage uponillumination. The current source in a solar cell is the charge carriersthat are created by the absorption of photons.

CIGS-Based Solar Cells

Copper ternary chalcogenide compounds and alloys are efficientlight-absorbing materials for solar cell applications. Their efficiencyis due to their direct and tunable energy band gaps, very high opticalabsorption coefficients in the visible to near-infrared (IR) spectrumrange and high tolerance to defects and impurities. Copperindium-gallium-selenium/sulfur (CIGS) thin film solar cells provide theadvantages of low-cost, high-efficiency, long-term stability, superiorperformance under weak illumination, and desirable resistance toradiation. For useful background material, refer, for example, to H. W.Schock et al., “CIGS-based Solar Cells for the Next Millennium,” Prog.Photovolt. Res. Appl. 8, 151-160 (2000).

Unlike the basic silicon solar cell, which can be described as a simplep-n junction device, CIGS based solar cells comprise a more complexheterojunction system. Solar cells based on CIGS have achieved thehighest efficiency of existing thin film solar cells.

A cross-sectional view of an exemplary CIGS device in accordance with aprior art embodiment is shown in FIG. 1. The various layers of the solarcell are deposited on a substrate 110. Solar cells based on p-type CIGSabsorbers are typically fabricated on glass, polymer, stainless steel orother substrates 110 using various deposition techniques known in theart. Incident sunlight 180 is partially blocked by the metallic gridshown as contact elements 170, which covers approximately 5% of thesurface of the device. These contact elements 170 can comprise Ni/Alfingers or other appropriate contact elements. The incident sunlight 180is partially reflected by the surface of the transparentconducting-oxide (TCO) layer 150 and 160, shown as a i-ZnO/ZnO:Al layer,due to the difference in the index of refraction. Some short-wavelengthphotons are absorbed in the n-CdS layer 140. Most of the sunlight,however, enters the semiconductor and is absorbed in the CIGS absorberlayer 130. The CIGS absorber layer 130 is shown disposed on molybdenum(Mo) layer 120, which is disposed on a stainless steel substrate 110 asshown. Other substrates known in the art are expressly contemplated,including glass and polymers.

The front metal contact fingers (Ni/Al) 170 are optional and are notrequired for operation of the photovoltaic device. The ZnO layers150&160 and CdS layer 140 typically comprise n-type material, and theCIGS 130 layer typically comprises p-type material. The semiconductingjunction is formed at or proximate to the CdS-CIGS (n-p) interface.Electrons that are generated within the junction-field region or withinabout one diffusion length of the n-p junction will generally becollected.

According to standard prior art CIGS thin film solar cells, a highestefficiency of 19.9% has been achieved with an effective area of 0.42 cm²prepared by the so-called three-stage co-evaporation process. For usefulbackground material on efficiencies of CIGS solar cells, refer, forexample, to I. Repins et. al., “19.9% efficient ZnO/CdS/CuInGaSe₂ solarcell with 81.2% fill factor”, Prog. Photovolt: Res. Appl. 2008;16:235-239.

QD-Enhanced Solar Cells

The efficiency of a solar cell can also be enhanced when a quantum dot(QD) effect is applied to a solar cell, thereby significantly improvingthe energy conversion rate thereof. Quantum dots have bandgaps that aretunable across a wide range of energy levels by changing the size (i.e.diameter) of the quantum dots. The quantum dot effect achieved by aquantum dot solar cell generally relates to an impact ionization effectand an Auger recombination (AR) effect.

The impact ionization effect occurs in semiconductor material, whenenergy of two bandgaps is provided from external, excited electrons canexist in form of hot electrons. When the hot electrons are transitedform high energy level to low energy level excitation state, thereleased energy can excite another electron from a valence band to aconduction band, and such phenomenon is referred to as the impactionization effect. According to the impact ionization effect, one highenergy photon can excite two or more hot electrons.

The other effect is an Auger recombination (AR) effect relative to theimpact ionization effect. The AR effect refers to the energy released inthe semiconductor material due to recombination of hot electrons andholes can excite another hot electron to transit to a higher energylevel, thereby prolonging a lifetime of the hot electron in theconduction band.

When the semiconductor material displays a quantum dot size, thecontinuous conduction band is gradually split into small energy levels,so that the cooling speed of the electrons is slowed down, and thereforethe impact ionization effect and the Auger recombination effect can beeffectively utilized. According to theoretical calculations, thetradition single junction solar cell only can achieve 31% energyconversion efficiency, and if combining with the impact ionization andAuger recombination effects, the maximum theoretical efficiency of thesolar cell can be 66%, which confirms the potential ability to use thequantum dots in the solar cell.

Solar cells made from photosensitive nanoparticles show very lowefficiencies. Nanoparticles are very efficient in generatingelectron-hole charge pairs when exposed to sunlight. The primary reasonfor the low efficiencies is charge recombination. To achieve highefficiencies in a solar cell the charges are desirably separated as soonas possible after they are generated. Charges that recombine do notproduce any photocurrent and hence do not contribute towards solar cellefficiencies. Charge recombination in nanoparticles is primarily due totwo factors: (a) Surface states on nanoparticles that facilitate chargesrecombination and (b) Slow charge transport. In nanoparticles/QD solarcells, the charge recombination is faster as compared to chargetransport.

Electricity produced by a solar cell is expensive due to high solar cellmodule cost. In order to significantly reduce the cost of solarelectricity, it is desirable to both increase cell efficiency and tosignificantly reduce the costs of photovoltaic (PV) module fabrication.A thin film form of cell reduces the fabrication cost, but yieldsrelatively lower efficiency compared to a single crystalline wafer basedcell. Thus, it is desirable to provide a system and method to enhanceefficiency of the solar cell.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art byproviding a photovoltaic (PV) device having a quantum dot sensitizedinterface and an absorber layer. In an illustrative embodiment, thephotovoltaic device includes a first conductor layer and a secondconductor layer, in which at least one of which is transparent to solarradiation. The quantum dot (nanoparticle) sensitized photo-harvestinginterface comprises a photo-absorber layer, a quantum dot layer and abuffer layer, placed between the two conductors.

The quantum dot sensitized interface PV device is fabricated orotherwise synthesized according to an illustrative procedure thatcommences by providing a substrate. A bottom electrode layer is thendeposited on the substrate, and an optical absorber layer is depositedon the bottom electrode. A quantum dot layer is formed on the opticalabsorber layer. A buffer layer is then deposited on the quantum dotlayer and a quantum dot layer is thereby formed at the interface of theoptical absorber and the buffer layer. The procedure then deposits toplight transparent electrode layers on the buffer layer to complete thePV device having a quantum dot sensitized interface. The resulting PVdevice provides improved efficiency over the prior art CIGS-based and/orQD-enhanced solar cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1A, already described, is a schematic cross-sectional view of acopper indium-gallium-selenium/sulphur (CIGS) thin film solar celldevice according to a prior art illustrative embodiment;

FIG. 1B, already described, is a band diagram of the CIGS thin filmsolar cell of FIG. 1A, according to the prior art illustrativeembodiment;

FIG. 2 is a schematic cross-sectional view of a quantum dot sensitizedthin film solar cell having a copper-based absorber layer, according toan illustrative embodiment;

FIG. 3 is a band diagram of a PN junction of the quantum dot sensitizedthin film solar cell of FIG. 2, showing the impact ionization effect,according to the illustrative embodiment; and

FIG. 4 is a flow diagram of a procedure for fabricating a quantum dotsensitized thin film solar cell according to the illustrativeembodiment.

DETAILED DESCRIPTION

A schematic cross-sectional view of a quantum dot (QD) sensitized thinfilm solar cell having a copper-based absorber layer is shown in FIG. 2,with the corresponding band-diagram shown in FIG. 3, according to anillustrative embodiment. With reference to FIG. 2, the QD sensitizedthin film solar cell includes a bottom electrode layer 220, opticalabsorption layer 230, buffer layer 250 and top light transparentelectrode layers 260 & 270 sequentially deposited on a substrate 210. Alayer containing a plurality of quantum dots 240 is formed at theinterface of the optical absorber layer 230 and the buffer layer 250.According to the illustrative embodiment, the QD layer comprisesphotosensitive nanoparticles. The absorption layer 230 comprises ap-type semiconductor layer and is in electrical communication with thebottom electrode layer 220. The photoactive QD layer 240 comprisesphotosensitive nanoparticles proximate the p-type absorber payer 230.The buffer layer 250 comprises an n-type layer, and is in contact withthe QD layer 240 and top transparent electrode layers 260, 270. The toptransparent electrode layers are in electrical communication with a topmetal contact 280.

The presence of QD layers at the PN interface 310 (see FIG. 3) and thestrong electric field created at the depletion region facilitateeffective separation of electron-hole pairs generated in the QDs (seebreak-out detail 320 of FIG. 3). In addition, the electric field drivesthe separated charges to their respective electrodes and minimizesrecombination of photo-generated carriers. Thus, as shown at 320,quantum size induced generation of multiple electron-hole pairs for asingle photon 325, through the impact ionization and the Augerrecombination effect, can be effectively separated and collected at therespective electrodes. Accordingly, this discloses a yield of higherphotocurrent generation, resulting in solar cell efficiency greater thanthe efficiency observed employing prior art devices.

Quantum Dots (i.e. the nanoparticles) form a schottky junction solarcell that does not yield high efficiency by itself. Typically, accordingto the prior art, the nanoparticles are embedded in other semiconductormaterials, for improved utilization of QDs. For example, InAsnanoparticles are inserted in GaAs host material thin film. Current CIGSsolar cell gives its maximum efficiency of approximately 20% and itstheoretical efficiency limit is approximately 31%. Introduction of QD'sproximate the CIGS layer increases its theoretical efficiency limit toapproximately 66%. The CIGS-based solar cells can create oneelectron-hole (charge) pairs for a photon. With the introduction of QDs,the solar cell can create multiple electron-hole pairs for a photon andit can extend the life-time of the generated charges (as shown in detail320 of FIG. 3). The resulting solar cell has the ability to enhance thespectrum of light that used for energy conversion.

Referring back to FIG. 2, the optical absorber layer 230 can be formedaccording to a physical vapor deposition process (for exampleco-evaporation, sputtering and selenization, etc.), solution basedprocess (such as electro-deposition, nanoparticle ink coating, etc.),and other techniques known in the art. Given that the buffer layer 250is typically formed by chemical bath deposition, the optical absorberlayer 230 can be coated together with the QD layer 240 by chemical bathdeposition.

The optical absorber layer 230 is a copper-based substance selected froma group consisting of copper indium gallium diselenide (CIGS), copperindium diselenide (CIS), copper gallium diselenide (CGS), copper galliumditelluride (CGT), and copper indium aluminum diselenide (CIAS). Then-type buffer layer is a substance selected from a group consisting ofcadmium sulfide (CdS), zinc sulfide (ZnS), indium sulfide (In₂S₃), andother similar materials known to those skilled in the art.

A bandgap of the quantum dots 240 can cover an infrared (IR), a visiblelight and an ultraviolet (UV) bands of solar spectrum. Various types ofQD materials can be simultaneously used to increase a photon absorptionrange. For example, if the bandgap of the QD is in the IR range, thematerial of the quantum dots is one or a plurality of substance(s)selected from a substance group consisting of PbS, GaSb, InSb, InAs andCIS, and other similar materials known to those skilled in the art.

If the bandgap of the quantum dots is in the visible light rage, thematerial of the quantum dots is one or a plurality of the substanceselected from a substance group consists of InP and CdSe etc. If thebandgap of the quantum dots is in the UV range, the material of thequantum dots is one or a plurality of substance selected from asubstance group consisting of TiO₂, ZnO and SnO₂ etc. Selection of thematerials of the quantum dots is highly variable and determines theconduction feasibility of the conduction band energy levels of theoptical absorber layer. The different types of materials can be usedsimultaneously to provide the desired coverage of solar spectrum.

Reference is now made to FIG. 4 showing an illustrative procedure 400for fabricating a PV device having a quantum dot sensitized interfacewith an optical absorber layer as shown and described herein inaccordance with the illustrative embodiment. The steps of the procedure400 correspond to the various steps in fabricating a QD sensitized thinfilm solar cell as shown in FIG. 2 and described herein. Accordingly,the layers comprise similar materials as described and are synthesizedusing techniques as described herein and known in the art. It isexpressly contemplated that the steps can be performed in any orderwithin ordinary skill to achieve the overall PV structure in accordancewith the illustrative embodiments herein.

As shown, the procedure 400 commences at step 410 by providing asubstrate. The substrate can comprise glass, polymer, stainless steel,or other substrates known in the art for use in solar cell applicationsand devices. According to the illustrative procedure, a bottom electrodelayer is deposited on the substrate at step 412. The bottom electrodelayer can comprise Mo (Molybdenum) or another appropriate layer that isdeposited on the substrate. Then at step 414 an optical absorber layeris deposited on the bottom electrode layer. The optical absorber layercan comprise CIGS or any other appropriate optical absorber as describedherein. At step 416, a QD layer is formed on the optical absorber. TheQD layer can be coated by chemical bath deposition or another techniqueknown in the art. A buffer layer is deposited on the optical absorberlayer at step 418. The buffer layer can comprise a CdS layer, whichallows some of the short-wavelength photons to be absorbed therein,according to illustrative embodiments. The buffer layer is typicallyformed by chemical bath deposition. Other techniques within ordinaryskill can also be employed to achieve the overall PV cell structure.Finally, at step 420, top light transparent electrode layers aredeposited on the buffer layer. The resulting thin film solar cell, forexample as shown in FIG. 4, provides a higher yield of photocurrentgeneration, resulting in solar cell efficiency greater than theefficiency observed by prior art devices.

The foregoing has been a detailed description of illustrativeembodiments of the invention. Various modifications and additions can bemade without departing from the spirit and scope of this invention. Eachof the various embodiments described above may be combined with otherdescribed embodiments in order to provide multiple features.Furthermore, while the foregoing describes a number of separateembodiments of the apparatus and method of the present invention, whathas been described herein is merely illustrative of the application ofthe principles of the present invention. For example, the illustrativeembodiments can include additional layers to perform further functionsor enhance existing, described functions. Likewise, while not shown, theelectrical connectivity of the cell structure with other cells in anarray and/or external conduit is expressly contemplated and highlyvariable within ordinary skill. More generally, while some ranges oflayer thickness and illustrative materials are described herein, it isexpressly contemplated that additional layers, layers having differingthicknesses and/or material choices can be provided to achieve thefunctional advantages described herein. In addition, directional andlocational terms such as “top,” “bottom,” “center,” “front,” “back,”“above,” and “below” should be taken as relative conventions only, andnot as absolute. Furthermore, it is expressly contemplated that varioussemiconductor and thin films fabrication techniques can be employed toform the structures described herein. Accordingly, this description ismeant to be taken only by way of example, and not to otherwise limit thescope of this invention.

What is claimed is:
 1. A method for fabricating a photovoltaic (PV) device comprising the steps of: depositing a bottom electrode layer on a substrate; depositing a p-type CIGS optical absorber layer on the bottom electrode, the p-type CIGS optical absorber layer being free of quantum dots; depositing a photosensitive quantum dot (QD) layer on the optical absorber layer, the QD layer comprising a first group of quantum dots having a bandgap in a visible light range, a second group of quantum dots having a bandgap in an infrared range, and a third group of quantum dots having a bandgap in an ultraviolet range; depositing an n-type buffer layer on the optical absorber layer, the n-type buffer layer being free of quantum dots; and depositing top light transparent electrode layers on the buffer layer.
 2. The method of claim 1 wherein the substrate comprises one of glass, polymer and stainless steel.
 3. The method of claim 1 wherein the optical absorber layer is deposited according to a physical vapor deposition process or a solution based process.
 4. The method of claim 1 wherein the buffer layer and the QD layer are deposited by chemical bath deposition.
 5. The method of claim 1 wherein the QD layer comprises at least one of PbS, PbSe, GaSb, InSb, InAs, CIS, InP, TiO₂, ZnO and SnO₂.
 6. The method of claim 1 wherein the buffer layer comprises at least one of cadmium sulfide (CdS), zinc sulfide (ZnS) and indium sulfide (In₂S₃).
 7. The method of claim 1 wherein the photosensitive QD layer comprises nanoparticles that capable of generating multiple electron-hole pairs for a single photon.
 8. The method of claim 1 further comprising: depositing a metallic grid of contact elements. 